Various imaging techniques have been used in medical diagnosis, radiation therapy planning, surgery planning, and other medical procedures, such as X-ray photography, magnetic resonance imaging (MRI), computed tomography (CT), positron emission tomography (PET), etc. For example, the CT-based image-guided radiotherapy (IGRT) has been widely used in radiation therapy. Conventionally, a radiation therapy treatment plan (also referred to as a treatment plan) for a patient is generated before the treatment starts. The treatment plan may be delivered to the patient during several treatment fractions spread over a treatment period of multiple days. During the treatment period, one or more IGRT images (e.g., a CT image) may be used to determine and/or position a region of interest (ROI) (e.g., a cancer). However, an IGRT image generated based on low-dose projection data may show noise and/or artifacts (e.g., staircase artifacts). The artifacts may reduce the image quality and influence the results of diagnosis made on the basis of such an image. A high-dose IGRT scan may at least partially alleviate these problems but at the cost of exposing a scanned patient to too more radiation. It is desirable to provide systems and methods for generating a high-dose image of improved quality, based on a low-dose scan.